Endocrine Problems

I. Goiter

1. Hyperplastic goiter is the most common thyroid disorder in the foal.
2. Cause is ingestion of excess iodine by the dam in the form of kelp containing feed supplements or plant goitrogens (Brassica spp.).
3. Idiopathic hyperplastic goiter is reported in western Canada.
4. High levels of iodine inhibit thyroid function and foals with goiter may be hypothyroid or may have normal T3 and T4 levels

II. Thyroid Activity of Normal Neonatal Foal

1. The newborn foal has the highest levels of thyroid hormones (T3 and T4) of domestic animal species [1] (Table 1).
2. Thyroid stimulating hormone administered to 1 day old foals increases T3 in 3 hours by 50% and 16% increase in T4 at 6 hours [2].

III. Hypothyroidism

Occurs in three forms:

1. Hypometabolic [3]

1. Thyroid may be of normal size.
2. Incoordination, poor sucking and righting reflexes, hypothermia, depression, poor growth, rough hair coat, angular limb deformities, anemia, lipemia, and abnormal bone ossification.
3. Be aware that severe illness may non-specifically lower T3 and T4 but thyroid "function" remains normal ("euthyroid sick").
4. Thyroid hormone levels may be low.
5. Poor response to TSH stimulation.

2. Developmental Lesions [4,5]

1. Thyroid levels may be normal at the time the abnormalities are noted, and foals may be bright, alert and growing.
2. Ruptured common digital extensor tendons, forelimb contracture, mandibular prognathism and immature carpal or tarsal bones are seen.

3. Hypothyroid and Respiratory Insufficiency [6]

1. Weak, persistent hypothermia, poor suckle, hypoxemia, respiratory acidosis.
2. Large thyroid glands with large follicles containing eosinophilic colloid.
3. Diffuse atelectasis - Incomplete surfactant development.

4. Treatment - After signs have developed, if thyroid levels are normal, no treatment is indicated. If T3 and T4 are low, replacement by Cytobin ®(Norden) at 1 μg/kg PO daily or 20-50 μg/kg Synthroid® PO daily is indicated. Measure values at 1 week to avoid over-dosing. Irvine suggests dose calculation of T4 in μg/day = fractional turnover (0.22) X (Kg body wt. X 0.08) X plasma T4 in μg/L X 10 for oral administration [3].

IV. Hypoadrenocorticism

Is seen in foals which are premature or normal gestation but dysmature.

1. Clinical signs of a premature foal is described in detail previously (See Assessment of Maturity - Prematurity).
2. Hematology and clinical pathology: Abnormally low neutrophil to lymphocyte ratio (<0.5-1.0 without bands or immature neutrophils) is common. Hypoglycemia or hypoinsulinemia may also be present.
3. Plasma cortisol <30 ng/ml 2 hours after birth -normal values 120-140 ng/ml [7].
4. In healthy neonates significant elevation in cortisol concentration should be observed after the administration of low dose (10 μg) or high dose of ACTH (100 μg) [8].
5. Aqueous ACTH (Synachten® Cosyntropin®) 0.125 mg (0.003-0.006 mg/kg) IM produces only 28% increase in plasma cortisol and no change in neutrophil - lymphocyte ratio in foals with hypoadrenocorticism. Normal foals should have an increase of about 200% in plasma cortisol
6. In one study, a small percentage of ill neonatal foals exhibited HPA insufficiency after the administration of aqueous ACTH (Cosyntropin®) [9].
7. A different study showed an increased ACTH/cortisol ratio in non-surviving septic foals in comparison to surviving septic foals which could indicate hypothalamic-pituitaryadrenal axis dysfunction at the level of the adrenal gland in these animals [10].
8. Vasopressin and ACTH levels of septic neonatal foals showed increased plasma AVP (Vasopressin) and ACTH concentrations in septic foals that did not survive. Several septic foals had increased AVP: ACTH and ACTH: cortisol ratios, which indicate relative adenohypophyseal and adrenal insufficiency [11]. See also treatment of relative adrenal insufficiency in Assessment of Maturity - Prematurity, Prematurity.